Biomedical subjects

G K Martin

Publications and source records attributed to G K Martin.

At least 37 records · Page 2Linked to original sources

PubMed1995

Dependence of distortion-product otoacoustic emissions on primary levels in normal and impaired ears. II. Asymmetry in L1,L2 space.

Previous studies indicate that the amplitude of 2f1-f2 distortion-product otoacoustic emissions (DPOAEs), evoked by two tones of frequencies f1 < f2, demonstrates a complex dependence on the levels (L1 and L2) of the primary tones. In the present study, 2f1-f2 DPOAE amplitudes were measured over a wide range of L1 and L2 in normal human ears, allowing a systematic, level-dependent asymmetry of DPOAE amplitude in L1,L2 space to be characterized. The L1,L2 at which DPOAEs were largest was close to L1 = L2 at high stimulus levels, but moved monotonically toward L1 > L2 as stimulus levels decreased. A related observation was that DPOAE amplitude had a greater dependence on L1 and on L2. These asymmetries were quantified in normal human ears, and compared to the corresponding asymmetries apparent in data from animal models. Recent studies have demonstrated that the reduction of DPOAE amplitude by cochlear trauma is greater when L1 > L2 than when L1 = L2, suggesting that the reduction of DPOAEs by trauma demonstrates an asymmetry in L1,L2 space that is qualitatively similar to that of normative DPOAE amplitude. To investigate this issue, 2f1-f2 DPOAE amplitudes were measured over a wide range of L1 and L2 in rabbit ears pre- and postinjection of the ototoxic loop-diuretic ethacrynic acid. The results indicate that the asymmetry in L1,L2 space of the reduction of DPOAEs by trauma is both qualitatively and quantitatively similar to the asymmetry in L1,L2 space of normative DPOAE amplitude. Specifically, the L1 values that maximized normative DPOAE amplitudes for any specified L2 (or, equivalently, the L1 values that allowed L2 to be minimized for any specified normative DPOAE amplitude) also yielded the greatest reduction of DPOAEs by the diuretic. In humans, the L1 values that maximize normative DPOAE amplitudes for any specified L2 are well approximated by a simple equation, with parameters that vary with frequency and f2/f1. It is suggested that the L1,L2 values defined by this equation may be optimum for use in clinical applications.

Acoustic Stimulation↗

PubMed1995

Effects of ear-canal standing waves on measurements of distortion-product otoacoustic emissions.

At frequencies above 3 kHz, standing waves in the ear canal complicate calibration of stimulus sound-pressure levels (SPLs) for measurements of distortion-product otoacoustic emissions (DPOAEs). In the literature, two stimulus-presentation strategies have been used for DPOAE measurements. In the "in-the-ear adjustment" strategy, the voltage command to the speakers is adjusted to maintain a constant stimulus SPL across frequency at the DPOAE-measurement microphone. In the "iso-voltage" strategy, the voltage presented to the speakers is held constant across frequency, on the basis of the assumption that the frequency response of the speakers is approximately flat at the eardrum in the average human ear canal. Because of standing-wave effects, there are large, systematic but idiosyncratic differences of stimulus SPL between the two strategies. DPOAE-versus-frequency functions ("DPOAE audiograms") obtained using both stimulus-presentation strategies in the same ears are presented. The differences of stimulus SPL between the two strategies, and the associated differences of DPOAE amplitude, are described and quantified. Around frequencies of standing-wave minima at the DPOAE probe, the in-the-ear adjustment strategy resulted in smaller DPOAEs at high L1 = L2, but much larger DPOAEs at low L1 = L2, than did the iso-voltage strategy. For any L1, the DPOAE-amplitude differences between the two strategies varied systematically with L1-L2. At the stimulus levels used to construct previously published population norms for clinical applications (i.e., L1 > or = 65 dB SPL), there are only small differences of mean DPOAE amplitudes, and of the standard deviations of these means, between the two strategies.

Acoustic Impedance Tests↗

PubMed1994

Sensitivity of distortion-product otoacoustic emissions in humans to tonal over-exposure: time course of recovery and effects of lowering L2.

An important concern of industrial hearing-conservation programs is detecting the onset of noise-induced hearing loss. If it can be shown that otoacoustic emissions are sufficiently sensitive to reliably detect auditory fatigue and the permanent hearing loss that eventually develops, they could become an important part of the hearing-conservation test battery. The present study in humans was designed to examine the influence of overall primary-tone level and the effects of lowering the f2 primary on the sensitivity of distortion-product otoacoustic emissions (DPOAEs) to acoustic overstimulation. One ear from each of 14 subjects with normal hearing was exposed to a 105-dB SPL pure tone at 2.8 kHz for 3 min using a protocol consisting of distinct pre-exposure, exposure, and post-exposure periods. As a quantitative index of the functional status of the outer hair cells, 2f1-f2 DPOAEs were monitored systematically over time using four stimulus-test conditions consisting of either one of two levels of equilevel primary tones, or one of two levels of offset primaries, with L2 set 25 dB lower than L1. The overall finding was that the DPOAE protocol incorporating both the lowest level of stimulation and an f2-primary tone that was 25 dB below the level of the f1 stimulus [i.e., L1 (55 dB SPL) - L2 (30 dB SPL) = 25 dB] was most sensitive to the exposure effects. The results establish that DPOAEs elicited with unequal, in contrast to equal-level primaries, have comparable signal-to-noise ratios, but are considerably more sensitive to reductions in emission levels induced by exposure to short-lasting, moderately intense tones. The recovery of DPOAE amplitudes over the first 15 min post-exposure appeared to be roughly linear in log time and, in many cases, could be closely approximated by fitting a logarithmic curve to the post-exposure data. From these functions, the initial amount of loss (y-intercept) and the slope of recovery were identified as potential measures of vulnerability to acoustic exposure in that these variables appeared to be related to the susceptibility of some of the subjects, who also participated in a subsequent experiment on the behavioral effects of the exposure stimulus. Finally, compared to behaviorally measured temporary threshold shift (TTS), the time course of the recovery for DPOAEs was very similar, suggesting that, with the appropriate parameters, DPOAEs can be as sensitive to TTS as routine pure-tone audiometry.

Acoustic Stimulation↗

PubMed1994

Cochlear endoscopy with preservation of hearing in guinea pigs.

Advances in fiberoptic technology have revolutionized the way many disorders are treated by otolaryngologists. We have previously described our experiences with cochlear endoscopy during cochlear implantation. However, endoscopy of the functioning cochlea has not previously been reported. To test the hypothesis that endoscopy of the guinea pig cochlea is possible without catastrophic loss of auditory function, we subjected 20 ears of 10 Hartley-strain albino guinea pigs to limited endoscopy of the cochlea through the round window with evaluation of distortion product otoacoustic emissions and auditory brain stem responses. Insertion of the endoscope caused measurable changes in auditory brain stem response latency and amplitude. Distortion product otoacoustic emission amplitudes were reduced an average of 6 dB with greater loss close to the round window. Frequencies above 18 kHz, corresponding to the region of endoscopy, were not evaluated in this preliminary study and are suspected to have sustained more damage. Results demonstrate that endoscopy of the guinea pig cochlea is possible without major loss of the above physiologic measures in the regions tested. If endoscopy of the cochlea is to become a tool with clinical and basic science applications, refinements in techniques to avoid damage are necessary.

Animals↗

PubMed1993

Distortion-product emissions in rabbit: I. Altered susceptibility to repeated pure-tone exposures.

An important issue in understanding the development of noise-induced hearing loss is whether prior acoustic overstimulation alters the susceptibility of the cochlea to further damage. The present work was designed to establish a model of activity-dependent changes in the susceptibility of the cochlea to acoustic overstimulation by regularly exposing the ear to a low-frequency pure tone. As a quantitative index of cochlear function, 2f1-f2 distortion-product otoacoustic emissions (DPOAEs) were monitored systematically over time in three groups of rabbits, with each group experiencing a unique paradigm that incorporated repeated exposure to the low-frequency tone. Common to each rabbit's exposure protocol was that a given experimental session consisted of two exposure episodes, separated by a 40-min period. Experimental sessions were repeated three times, with 2- to 3-day recovery periods interposed between sessions. The rate of decrement in DPOAE amplitude over a prescribed time period was utilized as a measure of susceptibility to the acoustic trauma. The overall results indicated that ears were more susceptible to exposure 40 mins after the first exposure of a session than they were initially. A series of control experiments indicated that the robustness of the acoustic middle-ear reflex (AMR) did not change between the exposure episodes. Consequently, changes in the AMR could not account for the increased susceptibility seen following the first exposure. However, in awake rabbits with stronger AMRs, higher pure-tone exposure levels were needed to produce increased susceptibility to the second exposure. After 2-3 days of intersession recovery, susceptibility to the effects of excessive sound returned close to its original baseline level. The outcome of these studies demonstrated a reduced capacity for the ear to resist the harmful effects of exposure to a moderately intense tone, which was repeated twice over a brief 40-min period, but little change in susceptibility when identical exposures were repeated over longer intersession intervals of several days.

Acoustic Stimulation↗

PubMed1993

Distortion-product emissions in rabbit: II. Prediction of chronic-noise effects by brief pure-tone exposures.

In an attempt to predict the susceptibility of the cochlea to the harmful influences of excessive sound, the effects of initially exposing the same rabbits to brief pure-tones were related to the subsequent effects of octave-band noise (OBN) exposure using measures of distortion-product otoacoustic emissions (DPOAEs). The pure-tone exposure paradigm consisted of determining the rate at which a 100-dB SPL, low-frequency tone reduced the amplitude of a 1.5-kHz DPOAE, elicited by 50-dB SPL primaries. To establish the stability of the rate-reduction index, the tonal-exposure protocol was repeated on three separate occasions for each subject. Subsequently, the same rabbits were exposed chronically to a 95-dB SPL OBN, centered at 1-kHz, until DPOAE amplitudes between 1-5 kHz were diminished to noise-floor levels, i.e., by 10-30 dB, in response to 45-dB SPL primaries. The results revealed a visually apparent relation between the slope of the tonal-induced DPOAE-loss functions and the number of days required to reach the criterion decrement in emission level during chronic exposure to noise. Analysis of the frequency extent of the noise-induced changes revealed significant correlations between the previously measured rate of pure-tone induced reductions of DPOAE amplitude and the subsequent amount of decrement produced by OBN exposure. Thus, rabbits exhibiting slow rates of tonal-induced decrements in low-frequency DPOAEs were resistant to the amplitude-reducing effects of a subsequent chronic OBN exposure in that it typically took more than four days to achieve the targeted amount of DPOAE loss.(ABSTRACT TRUNCATED AT 250 WORDS)

Acoustic Stimulation↗

PubMed1993

Distortion-product otoacoustic emissions in normal and impaired ears: insight into generation processes.

Otoacoustic emissions can be used to study cochlear function in an objective, non-invasive and rapid manner. These and other desirable features of emissions have inspired a significant amount of investigation into the practicalities of utilizing evoked emissions as clinical tests of hearing. Variables which affect the measurement of emissions can be sorted into two major categories consisting of factors affecting either emission generation or expression. The present report consolidates and summarizes recent findings of a series of experiments in our laboratory which address both the generation and expression of transiently evoked and distortion-product otoacoustic emissions. Because these two emission types have the greatest promise of becoming clinically useful, a complete understanding of the factors responsible for their measured properties is particularly important.

Animals↗

PubMed1993

The influence of noise on the measured amplitudes of distortion-product otoacoustic emissions.

This paper describes the influence of noise on the measured amplitudes of tonal signals, as determined using narrowband spectral analysis, that is, the technique typically used to measure distortion-product otoacoustic emissions (DPOAEs). At small signal-to-noise ratios, background noise results in overestimation of DPOAE amplitude and, thus, substantially influences the measured properties of the low-level portions of DPOAE-growth functions, in particular the apparent slope of the functions in this region. It is shown that, because of the influence of noise, the algorithm for the objective estimation of detection thresholds of DPOAEs, and of the slopes of DPOAE-growth functions, described by Nelson and Kimberley (1992), will tend to underestimate these values. This systematic underestimation is presumably the reason why many of the DPOAE-detection thresholds and growth slopes presented in that study were considerably lower than those reported in previous studies using similar measuring equipment and paradigms but different detection-threshold and growth-slope estimation techniques. In the present paper, a simple equation allowing an estimated correction for the effects of noise on measured DPOAE amplitudes is presented. Finally, an alternative strategy for the estimation of DPOAE thresholds, one that is less prone to the influence of noise, is suggested.

Acoustic Stimulation↗

PubMed1993

Clinical testing of distortion-product otoacoustic emissions.

Otoacoustic emissions have great promise for use in clinical tests of the functional status of outer hair cells, which represent cochlear structures that make a major contribution to the hearing process. A substantial literature is available concerning the evaluation of outer hair cell function by transiently evoked otoacoustic emissions. However, relatively little attention has been focused on the benefits of testing with distortion-product otoacoustic emissions. The purpose of this presentation is to provide knowledge of the principal advantages offered by distortion-product emissions testing.

Acoustic Impedance Tests↗

PubMed1993

Spontaneous otoacoustic emissions in different racial groups.

To determine if there are racial differences in the prevalence of spontaneous otoacoustic emissions (SOAEs), both ears of 20 Negro, 20 Asian and 20 Caucasian subjects were examined for the presence of SOAEs. Within each racial group, equal numbers of normally hearing males and females were tested. Significant differences in the occurrence of SOAEs were found between the three racial groups, with Negroes expressing more SOAEs than Caucasians, and Asians demonstrating an intermediate number of these emissions. In support of previous observations, more emissions were recorded from female than from male ears, and a significant correlation of the number of emissions in the two ears of an individual was also noted.

Acoustic Impedance Tests↗

PubMed1993

Otoacoustic emissions in children with normal ears, middle ear dysfunction, and ventilating tubes.

The clinical utility of otoacoustic emissions (OAEs) has been well established in adults. The purpose of this investigation was to determine the efficacy of OAE testing in children. Distortion-product OAE (DPOAE) audiograms, response/growth functions, and transiently evoked OAEs elicited with clicks were measured from the ears of both healthy volunteers, aged 4 to 13 years, and children with confirmed middle ear disorders. These measures established the means and variabilities for DPOAE and noise-floor amplitudes of normal and diseased young ears. Compared with adult emissions, the healthy young ears exhibited greater mean DPOAE and noise-floor amplitudes. In contrast, ears with type B and type C tympanogram patterns showed absent or markedly reduced OAE amplitudes, when compared with emissions measured in their control counterparts. Finally, ears with ventilating tubes exhibited OAE amplitudes lower than amplitudes from healthy ears, but higher than those of the untreated diseased ears. Although these findings imply that using OAEs to test the outer hair-cell reserve of infected ears is problematic, emitted responses provide useful information concerning the normalcy of middle ear function.

Acoustic Impedance Tests↗

PubMed1992

Test/retest reliability of distortion-product and transiently evoked otoacoustic emissions.

Otoacoustic emissions can be used to study cochlear function in an objective and noninvasive manner. These features of emitted responses have stimulated a great deal of investigation into the utility of evoked emissions as clinical tests of hearing. One practical and essential aspect of any clinical measure is the consistency of its result upon repeated testing of the same individual (i.e., its test/retest reliability). The goal of the present work was to conduct a systematic study of the test/retest reliability of the two evoked emission types, the transiently evoked and the distortion-product otoacoustic emissions, that have the greatest promise of becoming clinically useful. Toward this end, the short- and long-term reliabilities of these two response measures were examined in 12 normally hearing adults. The results of these experiments showed that the consistency of both measures of evoked otoacoustic emissions was generally excellent.

Adult↗

PubMed1992

Evidence for two discrete sources of 2f1-f2 distortion-product otoacoustic emission in rabbit: I. Differential dependence on stimulus parameters.

The results of studies of the physiological vulnerability of distortion-product otoacoustic emissions (DPOAEs) suggest that the DPOAE at 2f1-f2 in vertebrate ears is generated by more than one source. The principal aims of the present study were to provide independent evidence for the existence of more than one DPOAE source, and to determine the contributions of each to the ear-canal 2f1-f2 signal. To accomplish these aims, specific stimulus parameters were separately and systematically varied to provide detailed parametric information regarding 2f1-f2 DPOAE amplitude and phase in normal ears of awake rabbits. The findings indicate that two discrete sources, demonstrating differential dependence on stimulus parameters, dominate the generation of the 2f1-f2 DPOAE. One source of distortion is dominant above 60-70 dB SPL at moderate primary-frequency separations, and at all stimulus levels when the primary tones are closely spaced. The other source is dominant below 60-70 dB SPL at moderate primary-frequency separations, and may be dominant at all stimulus levels when the primary tones are widely separated in frequency. The results suggest that by varying stimulus parameters, it may be possible to independently study the two generator mechanisms.

Animals↗

PubMed1992

Evidence for two discrete sources of 2f1-f2 distortion-product otoacoustic emission in rabbit. II: Differential physiological vulnerability.

In a previous report, it was shown that, in normal rabbit ears, the amplitude and phase of 2f1-f2 distortion-product otoacoustic emissions (DPOAEs) elicited by low-level (< 60-70 dB SPL) stimuli display a differential dependence on stimulus parameters to those evoked by high-level (> 60-70 dB SPL) stimuli, indicating differences in the underlying generation mechanisms. In the present study, the physiological vulnerability of DPOAEs in each of the two 2f1-f2 DPOAE-response regions identified on the basis of differential parametric properties, was characterized. Thus emissions evoked using stimulus levels from 45-75 dB SPL were measured over time upon: (1) induction of lethal anoxia, (2) acute injection of ethacrynic acid, and (3) acute injection of ethacrynic acid 2 h after a single administration of gentamicin. The DPOAEs evoked by low-level stimuli (45 dB SPL) were abolished within 3-4 min of induction of anoxia, whereas DPOAEs evoked by high-level stimuli (75 dB SPL) were unchanged in this period. The high-level emissions decreased with a complex time course postmortem, and demonstrated behaviors, including evidence of susceptibility to fatigue, suggesting a dependence upon a cochlear energy supply. Low-level DPOAEs could be temporarily abolished, with complete recovery, by an acute administration of ethacrynic acid that had little effect on high-level DPOAEs. Treatment with the gentamicin and ethacrynic-acid combination, which would be expected to produce widespread hair-cell damage, eliminated low-level DPOAEs, and greatly reduced high-level emissions. In combination with previously published data, these findings strongly suggest that low- and high-level 2f1-f2 DPOAEs arise from discrete sources. The data are consistent with the proposal that the low-level DPOAE source is an active, micromechanical process, but suggest that the proposed origin of high-level DPOAEs exclusively in the passive macromechanics of the cochlear partition may be incorrect. The elimination of both low- and high-level DPOAEs revealed the presence of a third, residual 2f1-f2 DPOAE component, approximately 75-80 dB below the stimulus-tone levels, that may reflect the true passive-distortion response of the cochlea.

Acoustic Stimulation↗

PubMed1991

Effects of the crossed acoustic reflex on distortion-product otoacoustic emissions in awake rabbits.

Recent studies in anesthetized cats suggest that contralateral-sound stimulation acts to suppress ipsilateral neural responses via the medial olivocochlear-efferent system. Activation of this descending efferent pathway presumably influences ipsilateral outer hair cell motility and, thus, cochlear micromechanics, resulting in reduced input to auditory-nerve fibers. The principal aim of the present study was to determine if contralateral-sound stimuli influence the generation of ipsilateral distortion-product otoacoustic emissions, in the ears of awake rabbits. The results showed no effects of contralateral stimuli on these emissions that could not be attributed to the crossed acoustic middle-ear reflex. The findings further indicate that distortion-product otoacoustic emission amplitudes over a wide range of frequencies can be dramatically reduced when the middle-ear reflex is activated.

Acoustic Stimulation↗

PubMed1991

Slow variation of the amplitude of acoustic distortion at f2-f1 in awake rabbits.

A slow, reversible change in f2-f1 distortion-product otoacoustic emission (DPOAE) amplitude in the absence of correlated changes in 2f1-f2 DPOAE amplitude has been recently reported in anesthetized guinea pigs and gerbils, suggesting a dissociation in the generation of these two emissions. It was proposed that the observed changes in f2-f1 amplitude may represent an efferent influence on cochlear mechanics [Brown, A.M., Hear. Res., 34, 27-38, 1988]. In the present study, a search was made for similar changes in f2-f1 amplitude in awake rabbits. Qualitatively similar variations in f2-f1 DPOAE amplitude, in the absence of 2f1-f2 amplitude variations, were found in all ears tested, supporting the hypothesis of a dissociation of the generators of the two DPOAEs. The behavior of the f2-f1 DPOAE amplitude changes observed here, however, provided no evidence of an efferent involvement.

Acoustic Stimulation↗

PubMed1991

Altered susceptibility of 2f1-f2 acoustic-distortion products to the effects of repeated noise exposure in rabbits.

Hearing sensitivity and the generation of acoustic-distortion products at 2f1-f2 were examined systematically in behaviorally trained rabbits, before, during, and following regular exposure to a 95-dB SPL octave band of noise, centered at 1 kHz. During the exposure period, the octave-band noise was interrupted once every 24 h in order to monitor the progressive loss in auditory function using tests of behavioral threshold and distortion-product otoacoustic emissions (DPOAEs). When low-frequency DPOAEs from 1-4 kHz diminished to noise-floor levels, i.e., when their amplitudes were reduced by about 20-30 dB, the exposure was terminated. Subsequent recovery of behavioral thresholds and DPOAE amplitudes and detection 'thresholds' was evaluated at regular intervals over a 3-week post-exposure period. Following the recovery period, the rabbits again received the identical exposure/recovery treatment until a permanent 10 dB or greater loss in DPOAE amplitudes was achieved for any point of measurement between 2-10 kHz. The primary result was that the number of days of overstimulation required for rabbits to reach the criterion loss in DPOAE amplitudes increased for each successive exposure session. In addition, DPOAEs accurately tracked the frequency pattern described by the behavioral threshold shifts during both the development and recovery stages of exposure.

Animals↗

PubMed1991

Clinical applications of otoacoustic emissions.

On the basis of recent advances in auditory physiology, new tests of cochlear function have been developed using measures of otoacoustic emissions. In the present report, the clinical potential for each of the four basic emission types is examined. In addition, the practical advantages of examining the ear with two specific types of evoked emissions, transiently evoked and distortion-product otoacoustic emissions, are reviewed in detail. Finally, the future role of tests of otoacoustic emissions in the diagnosis of hearing impairment is discussed. The current view is the evoked emissions hold promise as an essential part of the clinical examination of the auditory system.

Audiometry↗